If two or more accumulators are connected in series and this series circuit is treated as a unit, in particular during charging, problems can arise which lead to the premature failure of the accumulators. These problems are to be attributed to the fact that, as a consequence of manufacturing tolerances, each accumulator has its own electrical, chemical and thermal characteristics.
In particular, as a consequence of scatter between examples, different charging end currents can arise during charging of the accumulators and different accumulator temperatures caused by the construction of the apparatus can lead to different states of charge of the accumulators and to the overcharging of individual accumulators when the series circuit of accumulators is treated as a unit of identical accumulators.
In addition to this, problems also arise during discharging of the accumulators, since individual accumulators or accumulator cells do not all make available the required capacity or cannot all make available the required capacity. Such accumulators or accumulator cells can be damaged by being fully discharged or by change of polarity.
These problems lead, in particular with closed accumulators, for example with so-called drysafe-accumulators, to failures, since these accumulators cannot be topped up with water.